Autothermal reforming is a very promising alternative to classic steam reforming for hydrogen production. In the reactor, an oxygen/water mixture reacts with hydrocarbon CnHm, without an external heat source, according to the following equations:CnHm+n H2O→n CO+(m/2+n)H2 ΔHR>0(Steam reforming)CnHm+n/2 O2→m/2 H2+n CO ΔHR<0(Partial oxidation)
For methane CH4 (n=1, m=4), the reaction equations are as follows:CH4+H2O=>CO+3H2 ΔHR=+206 kJ/molCH4+0.5 O2=>CO+2H2 ΔHR=−35 kJ/mol
As a rule, the oxygen is provided from air. The heat that is necessary for the steam reforming is provided by partial oxidation of the hydrocarbon. Thus the process can be conducted in an autothermal operating mode. In principle there is the potential for high efficiency because system-related enthalpy losses are only possible through the warm product gas stream. Autothermal reforming appears very promising, especially for the use of fuel cell systems for a vehicle drive with gasoline or diesel fuel as the fuel. This can be attributed to the high reaction temperature (approx. 800° C.) and good reaction kinetics.
In addition to the development of suitable catalysts for autothermal reforming of middle distillates, the utility of a reformer is largely dependent on whether operating conditions can be optimized. Reforming liquid fuels places great demands on the preparation of the educts before they enter the reaction zone of the reactor, i.e. the reformer.
A poor quality educt mixture normally has a negative effect on the conversion of the fuel since carbon black and so-called “hot spots” form in the reaction zone. In order to avoid this problem it is in particular important that the O22/C ratio and H2O/C ratio in the mixture remain as constant as possible and do not fluctuate. Sometimes carbon black even forms during production of the educt mixture and deposits in the mixing chamber.
The mixing chamber of a reformer therefore has the following functions:                Supplying the fuel        Atomizing and evaporating the fuel        Forming the mixture (homogenizing the fuel concentration in the air/steam stream)        Homogenizing the flow distribution (flow speed profile)        
Known from WO 00/10911 is a mixing chamber in which fuel is evaporated using super-heated water steam and is mixed with air in a second area. It is disadvantageous that with such a mixing chamber it is very difficult to evaporate the high boiling components of some liquid fuels, such as diesel fuel and heating oil. If the fuel is evaporated only by directly exchanging heat with a hot gas, initially only the low boiling components evaporate. Since large quantities of energy are taken from the gas for this purpose, the gas temperature drops continuously so that it is no longer adequate for evaporating the high boiling components. Therefore it is generally not possible to completely evaporate complex fuels in this manner.
DE 198 60 308 A1 discloses a method for utilizing a fuel, in which method the so-called “cold flame” is used as a precisely defined exothermic reaction as the heat source for evaporating the liquid fuel. Disadvantageously, this method suffers from the risk of carbon black forming if a reaction occurs/ignites between the oxidant and the liquid fuel.
In U.S. Pat. No. 5,826,422 a portion of the fuel is combusted in order to generate the heat necessary for the evaporation. It is disadvantageous that carbon black also forms in this classic combustion method.